CPRI over switched Ethernet
Nigel Bragg – CTO
1
Introduction
Background
 “CPRI requirements for Ethernet Fronthaul” (802.1CM call, 24-11-15)
 (http://www.ieee802.org/1/files/public/docs2015/cm-CPRI-requirements-1115-v01.pdf)
suggests :- maximum FDV = 5 μs or 10% of the E2E latency
 FDV (Frame Delay Variation) == PDV, but in IEEE 802 language
 FDV tolerance as an attribute of transport for a CPRI stream requires
mechanisms for clock recovery and buffer control at the receiver :
 irrespective of the value of the maximum delay tolerance;
 buffer sizes are modest in a CPRI context – 50 μs @ 10G < 64 KBytes
 Is maximum FDV = 5 μs or 10% of the E2E latency the right choice ?
 where maximum E2E latency is implementation-dependent but ~ 100 μs
 This contribution suggests an alternative way of looking at this
specification issue.
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Functional Requirement
 It is believed that the root functional requirement is to hold the
total one-way latency between a RRH and a BBU to less than ~ 100 μs,
to allow correct operation of the hybrid ARQ mechanism :
 discussion with CPRI required to determine the exact latency details
Transport Delay
emit
Frame arrival jitter
maximum FDV
margin
time
readout
 This suggests that the parameter which ought to be specified is
Total Fronthaul Delay = Transport Delay + maximum FDV + margin
< 100 μs
 This allows a trade-off between Transport Delay and maximum FDV :
 in “very short reach” deployments, FDV requirements are much relaxed
 maximum possible reach can still be obtained when required, by the use of
precision network timing techniques.
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Multiple Fronthaul profiles seem to have merit
We show a simple use case :
 2.4 Gbps CPRI bandwidth per RRH (e.g. 3-sector, single 20 MHz carrier)
 10 Gbps trunking, so 4 x RRH streams supported per trunk,
 no pre-emption, and assume 1500 Byte packets, so unit of delay = 1.2 μs
 may not be realistic – a deliberate “stress test”
CPRI streams
RRH
X
10Gbps
FDV contribution (delay units)
HoL blocking
1
** “race condition”
3
CPRI streams
10Gbps
X
CPRI streams
X
1
3
BBU
1 = 3
3 = 9
12 = 14.4 μs
 In this scenario, an allowance of 5 μs for “margin” still leaves
80 μs (~ 16 km) available as a Transport delay budget

** the term “race” is used as in “Delays and PDV in an Ethernet Fronthaul Network”
(http://www.ieee802.org/1/files/public/docs2015/cm-farkas-delay-pdv-1215-v01.pdf )
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Next steps ?
Suggest to CPRI Cooperation that in a switched Ethernet Fronthaul
environment specification of the maximum RRH  BBU delay is the
preferred parameter :
 put Transport delay + FDV into the Ethernet Transport domain,
 and let that domain sort it out
Extend the discussion on joint calls.
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